Differential amplifiers, such as amplifiers or comparators, are widely used to amplify inputted signals and to output amplified signals. The differential amplifiers are employed in an input and output units of electronic appliances for consumer use or communication use. A differential amplifier is disclosed in Japanese Patent Application Publication JP2000-278053 (page 9, FIG. 10). Recently, with development of miniaturization of a semiconductor device, speeding up of a semiconductor integrated circuit has advanced. High speed operation is also required for amplifiers and comparators having a differential amplifier composed of a semiconductor integrated circuit respectively.
In order to achieve higher speed operation, gate lengths of a semiconductor device such as a MOS transistor composing a differential amplifier are formed shorter. In the case, shortening gate lengths causes a problem that a DC offset voltage of a differential amplifier circuit increases due to variations in gate length or in threshold voltage, which arises due to fluctuation of the process condition in manufacturing MOS transistors. In addition, when a feedback voltage is inputted to the differential amplifier circuit to reduce the DC offset voltage, inputting feedback voltage caused a problem that input and output impedance of the differential amplifier circuit can not be large.